Drivers using a major thoroughfare in Brooklyn and Queens will soon face a slower speed limit of 25 miles an hour, the latest step in Mayor Bill de Blasio's campaign to reduce pedestrian deaths.

A nearly 8-mile stretch of Atlantic Avenue, from the East River in Brooklyn to the Woodhaven neighborhood in Queens, will see new speed-limit postings and blue signs declaring the corridor a "slow zone" by the end of the month, city...